uketuke no hito ga totemo teinei datta node, ansinsimasita.

受付の人 literally means the person of the reception, but in natural English it means:

• the person at reception
• the receptionist
• the front-desk staff member

Here, 受付 means reception / front desk, and 人 means person.

The particle の is linking two nouns.

• 受付 = reception
• 人 = person

So 受付の人 means the person belonging to / associated with reception.

This is a very common use of の in Japanese: one noun modifies another noun.

Examples:

• 日本の車 = Japanese car / car from Japan
• 学校の先生 = school teacher
• 受付の人 = person at reception

Both が and は could be possible, but they give slightly different nuances.

In this sentence, が marks 受付の人 as the subject connected to the following description:

• 受付の人がとても丁寧だった = the person at reception was very polite

Using が here feels natural when presenting the reason for what happened next.

If you used は instead:

• 受付の人はとても丁寧だったので、安心しました。

this would sound more like as for the receptionist, they were very polite, so I felt relieved, with a bit more topic contrast or emphasis.

So が is a very normal choice here.

丁寧 is a na-adjective.

Na-adjectives do not conjugate like i-adjectives. To say they were something in the past, you use:

• 丁寧だった = was polite

Compare:

• 静かだ = is quiet
• 静かだった = was quiet

So:

• とても丁寧だった = was very polite

Because な is only used when a na-adjective directly modifies a noun.

For example:

• 丁寧な人 = a polite person

But in your sentence, 丁寧 is not directly modifying a noun. It is being used as the predicate:

• 人が丁寧だった = the person was polite

So you do not use な there.

This is a very common pattern in Japanese.

Before ので, the clause usually appears in its plain form, even if the whole sentence ends politely.

So:

• 丁寧だったので = because [they] were polite
• 安心しました = I felt relieved

This combination is completely normal:

• plain form in the reason clause
• polite form at the end of the sentence

You may also hear 丁寧でしたので, which is more formal and polite, but 丁寧だったので、安心しました is very standard and natural.

ので means because, since, or so.

It connects a reason to a result:

• 受付の人がとても丁寧だったので = because the receptionist was very polite
• 安心しました = I felt relieved

So the whole sentence means something like:

• Because the person at reception was very polite, I felt relieved.

Compared with から, ので often sounds a little softer, more explanatory, and less direct.

Both can mean because, but they feel slightly different.

• ので: softer, more explanatory, often sounds a bit more polite
• から: more direct, more straightforward

So:

• 受付の人がとても丁寧だったので、安心しました。 = Because the receptionist was very polite, I felt relieved.

• 受付の人がとても丁寧だったから、安心しました。 = I felt relieved because the receptionist was very polite.

Both are grammatical, but ので often fits well when giving a calm explanation.

安心しました comes from the verb 安心する, which means:

• to feel relieved
• to feel reassured
• to feel at ease

So 安心しました means:

• I felt relieved
• I was reassured
• I felt at ease

In this sentence, the idea is that the speaker became less worried because the receptionist was polite.

Because the speaker is talking about a completed situation:

• the receptionist was polite
• as a result, the speaker felt relieved

So both parts are in the past:

• 丁寧だった = was polite
• 安心しました = felt relieved

Even in English, we would usually use the past here too:

• The receptionist was very polite, so I felt relieved.

とても means very.

It modifies 丁寧:

• 丁寧 = polite
• とても丁寧 = very polite

So:

• 受付の人がとても丁寧だった = The person at reception was very polite.

Yes, it is very natural.

It sounds like something someone would really say when describing an experience at a hospital, office, hotel, clinic, or other place with a reception desk.

A natural English translation would be:

• The person at reception was very polite, so I felt relieved.
• Because the receptionist was very polite, I felt at ease.

The Japanese sounds polite, normal, and smooth.

Not always exactly, but often yes.

受付の人 literally means the person at reception, so it focuses on where the person is / what role they had in that situation.

Depending on context, natural English could be:

• the receptionist
• the person at the front desk
• the person at reception

If you want a slightly more literal understanding, the person at reception is safest. If you want a natural job-title translation, the receptionist is often fine.